Amphibious aircraft are designed to land either on water or on land. Amphibious aircraft may typically be of two types, namely those with a pair of pontoons; and true flying boats with a water-tight hull. For fixed-wing aircraft, whether using skis or a hull, there will be a forward, or leading, direction, and a rearward or trailing direction.
In either instance it is possible to land, or to attempt to land, a fixed wing amphibious aircraft with the landing gear in an inappropriate configuration. That is, it is possible to make a landing on terrain with the wheeled landing gear retracted; and it is possible to make a landing on water with the wheeled landing gear extended. One cause of accidents in amphibious aircraft is landing in water with the wheels down. A water landing with wheeled gear extended may be catastrophic: as the gear descends into the water, it may tend to function as an oar or paddle that generates a substantial overturning moment on the aircraft, such as may tend to flip the aircraft over on its nose (if the gear plunges into the water symmetrically) or to spin and flip the aircraft, possibly in cartwheel fashion, if one wheel catches the water before the other. In either case the result may be very significant damage or personal injury, or both.
Alternatively, when the aircraft lands on terrain with the wheeled landing gear retracted, the effect may be that of a belly landing, with the possibility of doing significant damage to the aircraft hull (or pontoons). Damage may similarly occur if one or another wheel of the landing gear extends, while one or more other wheels remain retracted.
As amphibious aircraft may, by their nature, fly to destinations that may have non-optimal field conditions, retrieving a damaged aircraft after an unintentional gear-up landing may itself become an adventure.